In the prior art, the screw selection device includes a machine table, a driving means, a material selection device, a pinhole depth selection device and a camera detector. The material selection device, pinhole depth selection device and camera detector are installed on the machine table for detecting and selecting screws passing through the machine table. Other than the camera detector, all the detecting units are mechanical controlled. For example, the pinhole depth detecting unit uses a probe to detect the pinhole depth of a screw nut to determine whether it matches to a standard value.
In above mentioned prior art technology, the screw image is captured from a lateral side of the machine table by the camera detector. In photograph, the nut of the screw is supported by a receiving hole at an edge of a material receiving ring. However, this is only suitable for larger or longer screws and is not suitable for small screws which is commonly used in electronic products. This is because the small screws can not protrude out of the receiving hole for image capture. As a result, auto-selection and detection can not be performed.